Main-Chain Dynamics of Cardiotoxin II from Taiwan Cobra (Naja naja atra) as Studied by Carbon-13 NMR at Natural Abundance:  Delineation of the Role of Functionally Important Residues^†

Cardiotoxin analogue II (CTX II) is an all β-sheet, small molecular mass (6.8 kDa), basic protein possessing a wide array of biological properties. Nearly complete assignment of the protonated carbon resonances has been achieved by heteronuclear NMR experiments. The study shows that the correlation between the carbon-13 chemical shifts and CTX II structure is good in general, but interesting deviations are also noticed. To characterize the internal dynamics of CTX II, longitudinal, transverse relaxation rates and heteronuclear ¹³C{¹H} NOEs were measured for α-carbons at natural abundance by two-dimensional NMR spectroscopy. Relaxation measurements were obtained in a 14.1 T spectrometer for 50 residues, which are evenly spread along the CTX II polypeptide chain. Except for five α-carbons, all data were analyzed from a simple two-parameter spectral density function using the model free approach of Lipari and Szabo. The microdynamical parameters (S², τ_e, and R_ex) were calculated with an overall rotational correlation time (τ_m) for the protein of 4.8 ns. For most residues, the α-carbons exhibit fast (τ_e < 30 ps) restricted libration motions (S² = 0.79−0.89). The present study reveals that the functionally important residues located at the tips of the three loops are flexible, and the flexibility of residues in this region could be important in the binding of cardiotoxins to their putative “receptors” which are postulated to be located on the erythrocyte membrane. In addition, the results obtained in the present study support the earlier predictions on the relative role of the lysine residues in the erythrocyte lytic activity of cardiotoxins.